The present invention relates to processes for the preparation of polymers, including homopolymers, random copolymers, block copolymers, functionally activated polymers, and the like, and more specifically, to a polymerization process and to the polymers formed thereby. In embodiments, the present invention relates to a stable free radical moderated process for generating a thermoplastic polymer resin or thermoplastic resins with narrow polydispersities, that is, narrow polymer molecular weight distributions as indicated by the ratio M.sub.w /M.sub.n, where M.sub.w is the weight average molecular weight and M.sub.n is the number average molecular weight, and easily controllable modality from at least one monomer compound comprising heating for an effective period of time a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound in the presence of carbon dioxide, especially supercritical carbon dioxide modified with sulfur dioxide under conditions such that all polymer chain formations are initiated at about the same time; cooling the mixture to effectively terminate the polymerization; isolating the thermoplastic resin product; and optionally but preferably washing and drying the polymer resin product. With the processes of the present invention in embodiments, the temperature can be, for example, from about 100.degree. C. to about 160.degree. C., about 120.degree. C. to about 140.degree. C., and preferably from about 120.degree. C. to about 130.degree. C. The polymer resins generated by the process of the present invention in embodiments are essentially monomodal, and in embodiments by repeating the heating and carbon dioxide steps, that is, the combined initiation and polymerization step, there is provided a means for obtaining mixtures of monomodal polymer resins that are compositionally the same resin type having characteristics of both narrow polydispersity, and known or selectable modality. In embodiments, the process of the present invention provides a means for conducting bulk or neat free radical polymerization processes on multikilogram or larger scales. The aforementioned embodiments may be accomplished in a one or single pot reactor environment. Further, in embodiments polymeric chain growth proceeds by a pseudoliving mechanism and can provide thermoplastic resins of variable molecular weights from very low to very high, for example less than about 2,000 up to about 300,000, or greater, while maintaining narrow molecular weight distributions or polydispersities of, for example, about 1.05 to about 1.95, and wherein the monomer to polymer conversion is high, for example at least about 50 percent, and more specifically, from about 50 to about 99 to 100 percent. Moreover, in embodiments block copolymers can be synthesized by the aforementioned stable free radical moderated free radical polymerization processes, wherein each block formed is, for example, well defined in length by the reacted monomer, and wherein each block formed possesses a narrow molecular weight distribution, and wherein the block copolymer is substantially 100 percent block copolymer and not contaminated with the formation of homopolymer of the second monomer. The formation of the homopolymer of the second block monomer is a possible competing reaction which occurs in other prior art processes such as in Otsu's iniferter prior art mentioned herein.
One method to prepare polymers or copolymers having a narrow molecular weight distribution or polydispersity is by anionic processes. The use and availability of resins having narrow polydispersities in industrial applications is limited primarily since anionic polymerization processes are performed in the absence of atmospheric oxygen and moisture, require difficult to handle and hazardous initiator reagents, and consequently, such polymerization processes are generally limited to small batch reactors. In addition, the monomers and solvents that are used must be of high purity and anhydrous rendering the anionic process more costly than alternatives which do not have these requirements. It is, therefore, desirable to provide a free radical polymerization process that would provide narrow molecular weight distribution resins without the disadvantages of the aforementioned anionic polymerization processes.
Free radical polymerization processes are chemically less sensitive to impurities in the monomers or solvents typically used, and are completely insensitive to water. There, however, has been a need for an economical free radical polymerization process which is suitable for preparing narrow polydispersity resins by suspension, solution, bulk or neat, emulsion and related processes, and which polymerization process provides resins that can undergo further reaction to provide a number of resins, especially thermoplastic resins.
Copolymers prepared by free radical polymerization processes inherently have broad molecular weight distributions or polydispersities, generally greater than about four. One reason is that free radical initiators have half lives that are relatively long, from several minutes to many hours, and polymeric chains are not all initiated at the same time, and wherein the initiators provide growing chains of various lengths at any time during the polymerization process. Also, the propagating chains in a free radical process can react with each other in processes known as coupling and disproportionation, both of which are chain terminating reactions, thus chains of varying lengths are terminated at different times during the reaction process which results in resins comprised of polymeric chains which vary widely in length from very small to very large. For a free radical polymerization process to be effectively enabled for providing narrow molecular weight distributions, then all polymer chains are to be initiated at about the same time and premature termination by coupling or disproportionation processes must be minimized or avoided.
Otsu et al., in Makrotool Chem., Rapid Commun., 3, 127 (1982), discloses the use of iniferters as a means of producing block copolymers by a free radical polymerization process. A mechanism proposed for the reaction suggested that a pseudoliving propagating free radical chain exists and that it continues to grow with time. There are two primary major drawbacks associated with using iniferters. Iniferters tend to react very slowly and the percent conversion or degree of polymerization of monomer to polymer is low, for example about 40 percent even after 20 hours of reaction time; and the free radical trap that caps the end of the growing chain has the ability to initiate new chains at any time during the course of the reaction, see for example S. R. Turner, R. W. Blevins, in Polymer Reprints, 29(2), September 1988. This initiation results in new chains being initiated at various times during the polymerization and consequently results in a broadening of the polydispersity. The processes of the aforementioned Otsu reference are not believed to be applicable to the synthesis of narrow molecular weight distribution resins, particularly for polymers with high molecular weights.
The use of stable free radicals are known as inhibitors of free radical polymerizations, see for example, G. Moad et al., Polymer Bulletin 6, 589 (1982). Studies by, for example, G. Moad et al. J. Macromol. Sci.-Chem., A17(1), 51(1982) have reported on the use of stable free radicals as inhibitors of free radical polymerizations performed at low temperatures, for example below 90.degree. C. Little is known concerning the reaction of stable free radical agents at higher temperatures and at high monomer to polymer conversions.
In a hypothetical free radical polymerization of styrene in which chains are continually initiated over the course of the polymerization, and where chain termination by coupling processes is also occurring, calculations as described in, for example, G. Odian, Principles of Polymerization, pages 280 to 281, 2nd Ed., John Wiley & Sons, 1981 have shown that the narrowest polydispersity that one can theoretically obtained is 1.5. In practice, polydispersities much greater than 1.5 are actually obtained. Wide polydispersities of between 2.0 and 2.4 are typical for free radical homopolymerizations of styrene. With copolymer systems, polydispersities of greater than 4 are generally obtained. The stable free radical polymerization processes of the instant invention enable narrow polydispersities of between about 1.05 to about 2, and specifically about 1.1 to about 1.3 for polystyrene, and as low as 1.5 for various copolymers like styrene acrylates. Also, the stable free radical polymerization systems of the present invention permit polydispersities that are comparable to those obtained in anionic polymerizations.
U.S. Pat. No. 4,581,429 discloses a free radical polymerization process which controls the growth of polymer chains to provide primarily short chain or oligomeric homopolymers and copolymers including block and graft copolymers. The process employs an initiator reaction product having the formula (in part) =N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally, for example, from about 2,500 to about 7,000 having polydispersities generally of about 1.4 to about 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates, use relatively low reaction temperatures of less than about 100.degree. C., and use multiple stages. Reference to the working Examples of this patent indicate temperatures of less than 100.degree. C., one M.sub.w /M.sub.n ratio of apparently 1.15 (if the polymerization was allowed to continue similar to the other Examples, it is believed that the polydispersity would probably broaden and be greater than 1.15), and wherein the M.sub.n was 3,200 and the conversion was low, 1.4 to 1.8, and wherein the calculated nonreported conversion rates are low, for example 22 percent or lower. With the aforementioned processes, it is believed that thermoplastic polymers were not obtained. In Example 23, where there was an attempt to increase the degree of polymerization up to n=70, the temperature was increased to 120.degree. C. for 1.5 hours and there resulted a low molecular weight polymer of M.sub.n =6,700 and a broad polydispersity of 1.82. In Example 25, there was employed additional heating at 140.degree. C. for 2 hours to increase the degree of polymerization up to 22 which is still low and not in the region for the material to be considered a polymer. No molecular weight data was given in Example 25. Also, in Example 29 the mixture was heated to 120.degree. C. for 0.5 hours and n was only 11.
In U.S. Pat. No. 5,322,912, the disclosure of which is totally incorporated herein by reference, there is illustrated a free radical polymerization process for the preparation of a thermoplastic resin or resins comprising:
heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form the thermoplastic resin or resins with a high monomer to polymer conversion; PA1 cooling the mixture; PA1 isolating the thermoplastic resin or resins; and PA1 washing and drying the thermoplastic resin or resins, and more specifically, a free radical polymerization process for the preparation of a thermoplastic resin or thermoplastic resins comprising: PA1 heating at a temperature of at least 100.degree. C., and in embodiments from about 120.degree. to about 160.degree. C., a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizabte monomer to form the thermoplastic resin or thermoplastic resins with a high monomer to resin or resins conversion of at least about 50 percent and with polydispersity of from about 1.05 to about 1.95; and PA1 cooling the mixture. PA1 heating a mixture comprised of a free radical initiator, an oxo nitroxide stable free radical agent, at least one polymerizable acrylate monomer compound, and optionally a solvent to form a homopolymeric acrylate containing thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity. PA1 heating a first mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form a first intermediate product resin; PA1 cooling said first mixture; PA1 isolating said first intermediate product resin; PA1 adding to said first intermediate product resin a second mixture comprised of at least one polymerizable monomer compound, wherein said polymerizable monomer compound of said second mixture is different from said polymerizable monomer compound of said first mixture, to form a combined mixture; PA1 heating said combined mixture to form a third mixture comprised of a block copolymer thermoplastic resin comprised of a first product resin formed from said first intermediate product resin and added said second monomer; PA1 cooling said third mixture; PA1 isolating said block copolymer thermoplastic resin from said third mixture; and PA1 optionally washing and drying said block copolymer thermoplastic resin, and wherein said heating is accomplished at a temperature of from about 40.degree. to about 100.degree. C. in the presence of ultrasonic irradiation. PA1 heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound; PA1 adding under pressure a supercritical fluid; PA1 polymerizing said polymerizable monomer to form said thermoplastic resin or resins with a high monomerto polymer conversion; PA1 cooling said mixture; PA1 optionally isolating said thermoplastic resin; and PA1 optionally washing and drying said thermoplastic resin. PA1 heating a first mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound; PA1 adding under pressure supercritical carbon dioxide followed by the addition of sulfur dioxide; PA1 polymerizing said first polymerizable monomer to form a first intermediate product resin; PA1 extracting unreacted said first polymerizable monomer compound from said first mixture; PA1 optionally cooling said first intermediate product resin; PA1 adding to said first intermediate product resin a second mixture comprised of a free radical initiator, a stable free radical agent, and a second polymerizable monomer compound, wherein said polymerizable monomer compound of said second mixture contains the same components as said polymerizable monomer compound of said first mixture, and said free radical initiator and said stable free radical agent of said second mixture are the same or different from said free radical initiator and said stable free radical agent of said first mixture, and wherein there is formed a combined mixture; PA1 adding sulfur dioxide and adding under pressure supercritical carbon dioxide; PA1 heating said combined mixture to form a third mixture comprised of a mixture of thermoplastic resins comprised of a first product resin formed from said first intermediate product resin and added said second monomer and a second product resin formed from said second monomer; PA1 extracting unreacted said second polymerizable monomer compound from said third mixture; PA1 cooling said third mixture; PA1 optionally isolating said mixture of thermoplastic product resin from said third mixture; and PA1 optionally washing and drying said mixture of thermoplastic resins and wherein said first product resin and said second product resin each possess a narrow polydispersity; and a free radical polymerization process for the preparation of thermoplastic resin or resins comprising: PA1 heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound; PA1 adding under pressure a supercritical fluid, and mixing with sulfur dioxide; PA1 polymerizing said polymerizable monomer to form said thermoplastic resin or resins with a high monomer to polymer conversion; PA1 cooling said mixture; PA1 optionally isolating said thermoplastic resin or resins; and PA1 optionally washing and drying said thermoplastic resin, or resins.
U.S. Pat. No. 5,059,657 discloses a polymerization process for acrylic monomers by contacting the monomers with a diazotate, cyanate or hyponitrite, and N-chlorosuccinimide, N-bromosuccinimide or a diazonium salt. The polymer produced can initiate further polymerization, including use in block copolymer formation.
U.S. Pat. No. 5,312,871 discloses a radical polymerization process for the preparation of narrow molecular weight distribution polymers which involves polymerizing a vinyl monomer or monomers with an initiator or initiating system comprising an alkyl or aryl metal, a strongly binding monodentate, bidentate or polydentate ligand and a stable oxy free radical. The initiating system of U.S. Pat. No. 5,312,871 is very complex, consisting of three components; an alkyl or aryl metal and a binding ligand (monodentate, bidentate or polydentate material), plus the stable oxy free radical. Examples of the alkyl or aryl metal are triisobutyl aluminum, diisobutyl aluminum hydride, dichloro ethyl aluminum, diethyl zinc, butyl lithium and phenyl magnesium bromide. Examples of the binding ligands are triphenylphosphine, bipyridyl, dimethylglyoxime and porphyrin. Examples of stable oxy free radical are TEMPO and galvinoxyl. The polymerization process is typically performed in a solvent media such as benzene, toluene or hexane at tern peratures in the range of 0.degree. C. to 100.degree. C.
In free radical polymeriz ation reaction processes of the prior art, with the exception of the U.S. Pat. No. 5,322,912, various significant problems exist, for example difficulties in predicting or controlling the polydispersity and modality of the polymers produced. These free radical polymerization processes usually provide polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities, or oligomers. Further, bulk or neat free radical polymerization processes of the prior art are prone to generating excessive quantities of heat since the polymerization reaction is exothermic and as the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-Interscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distribution and/or difficulties encountered during filtering, drying and manipulating the product resin. These and other disadvantages are avoided, or minimized with the processes of the present invention.
Illustrated in U.S. Pat. No. 5,274,057, the disclosure of which is totally incorporated herein by reference, is that free radical suspension polymerization reactions may also lead to undesirable deposits of polymer on the agitator, baffles, heating coils and reactor walls. In some situations, the suspension coalesces during the reaction producing large deposits of undesirable polymeric gel material which is difficult, expensive and hazardous to remove from the reactor.
Illustrated in U.S. Pat. No. 5,412,047, the disclosure of which is totally incorporated herein by reference, is polymerization process for the preparation of homopolymeric acrylate containing thermoplastic resin or resins comprising:
Illustrated in copending application U.S. Ser. No. 348,022, the disclosure of which is totally incorporated herein by reference, is a free radical polymerization process for the preparation of a block copolymer thermoplastic resin or resins comprising:
Also mentioned are commonly owned and assigned copending applications U.S. Ser. No. 08/181,134, filed Jan. 4, 1994, now U.S. Pat. No. 5,401,804; U.S. Ser. No. 08/307,192, filed Mar. 25, 1993 now abandoned; continuation-in-part of U.S. Ser. No. 07/976,604, now U.S. Pat. No. 5,322,912 filed Nov. 16, 1992, U.S. Ser. No. 08/214,518, filed Mar. 18, 1994; and U.S. Ser. No. 08/223,418, filed Apr. 4, 1994.
The disclosures of the above mentioned patents, publications, and copending applications are incorporated herein by reference in their entirety.
The thermoplastic resin products of the present invention can be selected for a number of uses, such as toner, developers and more specifically, as toner resins for electrophotographic imaging processes or where monomodal or mixtures of monomodal narrow molecular weight resins or block copolymers with narrow molecular weight distribution within each block component are suitable such as in thermoplastic films and coating technologies.
In copending patent application U.S. Ser. No. 413,752, the disclosure of which is totally incorporated herein by reference, there is illustrated a free radical polymerization process for the preparation of thermoplastic resin or resins comprising: